During navigation when travelling towards a desired destination along a predetermined route at sea by boat, it is often important that this route is followed more or less precisely, partly for finding the right way up to the route destination and partly for avoiding grounding against any reefs and rocks under the water surface and the sea bed in ground water. For this purpose, maps are traditionally used in the form of nautical charts and optionally a compass, wherein the boat driver compares the map image with the reality and estimates visually where the boat is located on the nautical chart. If the driver does not already know the route “by heart”, this must be done more or less continuously in order to make the navigation along the route correct and safe. As is well known, this may be a difficult task, particularly when the visibility is deteriorated and at larger bays where it is difficult to orientate oneself, and when the driver needs to concentrate on operating the boat. Likewise during orientation in land terrain or in the air, it is necessary to read a map and compare the map image with the reality in order to find the correct route.
In order to navigate at sea, the well-known GPS (Global Positioning System) technique is used today to an increasing extent for determining the boat position in relation to the surrounding water and land. A GPS apparatus utilizes satellite signals for calculating the position, and nautical charts exist in the form of software to be installed in a computer which, using received GPS signals, can indicate on a viewing screen the boat position on the map image during the drive. Thereby, the driver is not completely dependent on being able to correctly interpret islands, land and seamarks in the surroundings and estimate distances, etc., but may to a great extent rely on this indication for orientation, which is a great aid particularly at deteriorated visibility such as darkness and fog.
Known navigation systems also exist having indication of how the course should be adjusted in order to follow a predetermined route, e.g. by means of visual indication of necessary course adjustment starboard/port (right/left).
US 2002/0072850 A1 discloses a GPS-based steering aid device for a mowing vehicle that must at all times keep a straight course along a desired route line PP, in order to produce a straight mowing row. An array of luminous diodes on a display 32 shows how great the deviation T of the vehicle is from the route line PP. Furthermore, the necessary course correction is shown by selecting at regular intervals a point of reference IC somewhere on the route line PP, towards which point the user should steer. Next, an angle S is calculated by which the vehicle should change its course in order to steer towards the selected point of reference IC, which is indicated by means of an array of luminous diodes on a display 30. Thus, the display 30 shows an angle of correction towards the selected point of reference IC, and not the steering error angle E.
Further, U.S. Pat. No. 4,811,679 A discloses a navigation system where an elongated appearance pattern is displaced laterally on a display depending on course deviations. DE 4412859 C1 discloses a navigation system for road vehicles where a direction arrow shows the direction of the next turnoff, and the distance thereto is indicated by means of a “bar diagram” in the shaft of the arrow. U.S. Pat. No. 4,117,482 A discloses another navigation system for a boat over a number of turning points on a route in a watercourse, where the distance to the next turnoff is shown as a numerical value (in foot) on a display 39.
However, it is a problem not knowing how great accuracy is required for maintaining the course during the drive. For example, during a boat drive over large open water areas at a safe distance away from islands and ground water etc., it is not that important to follow a route exactly, and in that case it may be desirable for the driver to reduce his/her attention. On the other hand, it is more important to follow a precise route in narrow passages and/or where ground water is present, which is not unusual even at apparently open waters, thus demanding relatively greater attention from the driver. However, the navigation systems known today do not provide any instructions on how great accuracy the course must be maintained with in different situations.
Hence, there is a need for an improved aid for navigation and orientation when following a predetermined route, with distinct and easily read visual indication of the correct course and of how great the demand is for course adjustments. This information should be easy to see and grasp in a correct manner, even by means of merely a swift glance, particularly when the driver's attention may be needed for focusing on other things.